1. Field of the Invention
The present inveniton relates to a drive control method for a thermal transfer printer adapted to carry out printing by melting a thermofusible ink of an ink ribbon by means of a thermal head and transferring the molten ink of the ink ribbon onto a printing paper. More particularly, the present invention relates to a drive control method for a thermal transfer printer adapted to carry out printing by using a color ink ribbon called a multicolor ribbon on which a plurality of ink layers having different colors are formed.
2. Description of the Prior Art
Referring to FIG. 2 which shows an example of a general thermal transfer printer, reference numeral 1 generally designates a frame of the thermal transfer printer. The frame 1 is provided at its substantially central portion with a flat platen 2 having a substantially vertical printing surface. A carriage shaft 3 is located before and under the platen 2 so as to extend along the platen 2 in parallel relationship thereto. A front end of the frame 1 is formed into a flanged carriage guide 4. An inside edge of the carriage guide 4 is formed into a rack 5. A carriage 6 is mounted on the carriage shaft 3 and the carriage guide 4. The reciprocates along the carriage shaft 3 and the carriage guide 4 by a motor driven gear (not shown in FIG. 2) meshing with the rack 5. A thermal head 7 is mounted on a front end of the carriage 6 so as to face the printing surface of the platen 2. A photosensor 28 is provided at a side portion of the carriage 6 for detecting a ribbon end indication mark formed at an end portion of an ink ribbon accommodated in an ink ribbon cassette (not shown) adapted to be mounted on an upper surface of the carriage 6 or detecting plural color indication marks for respectively indicating colors of plural ink layers formed on a color ink ribbon accommodated in an ink ribbon cassette (not shown) adapted to be mounted on the upper surface of the carriage 6.
A paper insert section 8 for admitting a printing paper (not shown) and guiding the same to the front side of the platen 2 is formed behind the platen 2. A plurality of paper feed rollers 9 for feeding the printing paper at a predetermined speed is provided at a front end of the paper insert section 8. A plurality of pressure rollers 10 adapted to be respectively pressed against the paper feed rollers 9 are provided below the paper feed rollers 9. Thus, the printing paper inserted from the paper insert section 8 is held between the paper feed rollers 9 and the pressure rollers 10, and is fed to the front side of the platen 2.
FIG. 3 shows a carriage driving mechanism, a thermal head pressing/releasing mechanism, and ribbon winding mechanism in such a printer as shown in FIG. 2. Referring to FIG. 3, a carriage driving motor 11 is provided in the carriage 6. An output of the carriage driving motor 11 is transmitted through a pinion 12 to a gear 13 meshing with the rack 5. Accordingly, the carriage 6 can be reciprocated along the carriage shaft 3 and the carriage guide 4 by normal rotation and reverse rotation of the carriage driving motor 11.
A thermal head mounting member 14 is pivotably supported at a base end thereof to a fulcrum 15 provided at a fore end portion of the carriage adjacent to the plates 2. The thermal head 7 is mounted on a free end portion of the mounting member 14. A cam follower 17 is pivotably supported at an intermediate portion thereof to a fulcrum 16 provided behind the mounting member 14. One end 17a of the cam follower 17 is in contact with the mounting member 14, while the other end 17b of the cam follower 17 is in slidable engagement with an endless cam groove 19 of a cam member 18 as a drive member. The cam member 18 is integrally formed with an outer peripheral gear 20. The gear 20 meshes with a transmission gear 21 which in turn meshes with an output pinion 23 of a thermal head actuator motor 22 mounted in the carriage 6. Accordingly, the cam member 18 is rotated by driving the motor 22. The rotation of the cam member 18 is followed by rotation of the cam follower 17. By the rotation of the cam follower 17, the mounting member 14 contacting with the one end 17a of the cam follower 17 is moved toward and away from the platen 2. As a result, the thermal head 7 presses the ink ribbon and the printing paper against the platen 2 (this condition will be hereinafter referred to as a head-down condition), or is released from the platen 2 (this condition will be hereinafter referred to as a head-up condition).
Further, an ink ribbon winding bobbin 24 for winding the ink ribbon is provided on the carriage 6, and a motor 25 for driving the ink ribbon winding bobbin 24 is mounted in the carriage 6 independently of the thermal head actuator motor 22. An ink ribbon winding gear 27 is provided in the carriage 6 in coaxial relationship with the winding bobbin 24 through a so-called slip mechanism. The winding gear 27 meshes with a transmission gear 26 which in turn meshes with an output pinion 25a of the motor 25. Further, although not shown, there is provided a sensor for detecting the head-down condition or the head-up condition of the thermal head 7. In response to the head-down condition or the head-up condition of the thermal head 7 detected by this sensor, the motor 25 is controlled in operation.
FIG. 4 shows a conventional color ink ribbon to be employed for color printing. Referring to FIG. 4, an elongated color ink ribbon 29 is formed with a plurality of repeating units of ink layers continuously arranged in a longitudinal direction of the ink ribbon 29. Each repeating unit of ink layers consists of yellow (Y), cyan (C) and magenta (M) ink layers. At the boundaries of the adjacent ink layers in each repeating unit, there are formed a plurality of color discrimination marks 29a, 29b and 29c for discriminating the colors Y, C and M. In this example shown in FIG. 4, the colors Y, C and M are discriminated from one another by differentiating the number of bars constituting the color discrimination marks 29a to 29c. These marks 29a and 29c are detected by the photosensor 28 mounted on the carriage 6, thereby detecting the colors Y, C and M formed on the color ink ribbon 29 and printing data in a desired color.
Drive control for the above-mentioned thermal transfer printer is carried out in the following manner.
The printing paper is first inserted from the paper insert section 8, and is held between the paper feed rollers 9 and the pressure rollers 10. Under the condition, the paper feed rollers 9 are rotationally driven to thereby feed the printing paper by a predetermined amount in a direction perpendicular to a moving direction of the carriage 6. Then, the ink ribbon winding motor 25 is driven to wind the ink ribbon 29 by an amount such that a desired color ink layer on the ink ribbon 29 comes to a print start position. Then, the thermal head actuator motor 22 is driven to press the thermal head 7 against the platen 2 under a predetermined pressure at a position slightly before the print start position. Under this condition, the carriage driving motor 11 is driven to move the carriage 6 and accordingly move the thermal head 7 toward the print start position. Simultaneously, the ink ribbon winding motor 25 is driven to rotate the winding bobbin 24 and wind the ink ribbon 29, while the thermal head 7 is driven according to a desired recording signal, thereby obtaining a desired color print on the printing paper.
In the case that the color is intended to be changed into a different color during the printing of one line, the movement of the carriage 6 is stopped at the end of the printing of the previous color. Then, the thermal head 7 is released from the platen 2. Thereafter, the ink ribbon winding motor 25 is driven again to rotate the winding bobbin 24 until a color discrimination mark corresponding to a is detected by means of the photosensor 28. When a desired color ink layer comes to a position opposed to the thermal head 7, the motor 25 is stopped. Then, the thermal head 7 is again pressed against the platen 2, and the carriage 6 is moved together with the thermal head 7 to a next print start position. Subsequently, the same printing operation as mentioned above is carried out for the current desired color. After the printing of the current one line is terminate, the carriage 6 is returned to an initial position in the next line to be printed. In this case, the paper feed rollers 9 are rotated to feed the printing paper by the predetermined amount simultaneously with the return operation of the carriage 6. However, in the case of printing the next data over previously printed data in the same line in different colors, the paper feeding operation is not carried out but only the return operation of the carriage 6 is carried out.
However, upon pressing the thermal head 7 against the platen 2 under a predetermined pressure with the printing paper and the ink ribbon 29 spaced from the platen 2, the ink of the ink ribbon is unnecessarily transferred onto the printing paper at the position slightly before the print start position. That is, at the position where the thermal head 7 is pressed, the ink layer of the ink ribbon 29 is present, causing unwanted transfer of the ink onto the printing paper at the head pressing position by the impact of the thermal head 7. As a result, the printing paper is stained with the ink transferred. Particularly, in the case that the printing paper is a wood-free paper having a smooth surface, such unwanted transfer of the ink at the head pressing position is considerably.